Reduction of oxides by ion bombardment



Au'g- 15, 196-7 w. N. MAYER 3,336,211

REDUCTION OF OXIDES BY ION BOMBARDMENT Filed April 30, 1963 Z-Sheets-Sheet 1 2 LANGMUIR SHEATH L 7 I AC. 32 i l OXYGEN +2OV.D.C. ABSENCE OF ELECTRONS BECAUSE OF FIELD 0N FACE OF TARGET 12 0c .6

INVENTOR.

WILLIAM N. MAYER W19- f I ATTO NEY Aug. 15, 1967' w. N. MAYER 3,336,211

REDUCTION OF OXIDES BY ION BOMBARDMENT Filed April 50, 1963 2 Sheets-Sheet 2 4OO TO 800 V. D. C. 36

FIG. 3

\ INVENTOR. WILLIAM N. MAYER ATTORNEY United States Patent 3,336,211 REDUCTION OF OXIDES BY ION BOMBARDMENT William N. Mayer, White Bear Lake, Minn., assignor, by mesue assignments, to Litton Systems, Inc., Beverly Hills, Califi, a corporation of Maryland Filed Apr. 30, 1963, Ser. No. 276,874 2 Claims. (Cl. 204-192) This invention relates to sputtering and more particularly to a method for ionic bombardment of an oxide body to provide a concentration of the oxidized material on a bombarded surface of the oxide body. Furthermore, it is directed toward a novel technique for forming a bond between an oxide base material and a layer of metal Ivavhich is deposited on the bombarded surface of the oxide ody.

Bonding a metal to an oxide has presented problems of accomplishing a complete and satisfactory bond between the metal and the oxide. The difficulty has arisen primarily because the metal does not form an integral part of the atomic structure of the oxide. Reasonably good bonds have been attained by utilizing a metal paint which is fired after it is placed on the oxide surface. This process of bonding a metal to an oxide does not, as previously noted, form a bond wherein the metal is a part of the atomic structure of the oxide. Such bonds are not as durable as might be desired and in some cases are extremely diflicult to accomplish.

It is therefore an object of the present invention to provide a new and improved method of sputtering the surface of an oxide.

' It is another object of the present invention to provide a new and improved method of removing oxygen from the surface of an oxide body by ion bombardment.

It is a further object of the present invention to provide a method of producing a concentration of the oxidized material on the surface of an oxide body or base.

Another object of the present invention is to provide a new and improved method of depositing a layer of metal on the surface of a metal oxidevto form a bond wherein -the metal is part of the atomic structure of the oxide.

A further object of the present invention is to provide a new and improved method of depositing a metal on a metal oxide surface by bombarding the oxide and the metal with ions generated in aglow discharge plasma. .The foregoing objectives are attained by providing a glow discharge plasma in which a body of oxide is positioned. An alternating potential is applied to the target electrode or oxide whereby the target is bombarded by positive ions. The oxygen atoms which are in combination with some other element in the oxide are removed by the bombarding positive ions, thus leaving a concentration of the oxidized element at the surface of the oxide body. Next, a metal body may be bombarded with the same ions as'the'oxide body to produce sputtered atoms of metal which will diffuse throughout the area occupied by the oxide and the metal body and will form a layer on the surface of the oxide body exposed to the diffused metal atoms. Thus a layer of metal is deposited on the oxide body and is combined in the atomic structure of the oxide.

Other objects and features of the invention will become apparent from the following description of an embodi- 'ice ment thereof taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic view of a bell jar showing the location of various electrodes utilized to develop a gaseous or glow discharge plasma for ion bombardment;

FIGURE 2 is a fractional view of the oxide body which is used as a target and which shows the portion of the oxide body from which the oxygen is discharged; and

FIGURE 3 is an embodiment of the invention wherein metal is deposited on the oxide body after it has been bombarded by positive ions to remove oxygen from the surface of the oxide.

Although the present invention is illustrated and will be described in conjunction with a low pressure mercury plasma tube, it is to be understood that it is not limited to any particular means for providing the glow discharge or to any particular gas utilized to provide the glow discharge plasma. The teachings of the invention may be applied with corresponding advantages and benefits to a plasma produced by a glow discharge, a high frequency discharge or any other means. Furthermore, the invention is not limited to the use of either a direct current or an alternating current discharge, although it is described as applied to a direct current discharge tube.

Certain environmental conditions must be controlled in order to generate an atmosphere or plasma in which sputtering of a target material such as an oxide may take place with any degree of efficiency. This atmosphere or plasma may be generated in a closed vessel such as a bell jar or similar closed container. Refer to FIGURE 1 for an illustration of one type of envelope which may be used to provide the proper atmosphere for sputtering a target material. An envelope 11 which is vacuum tight is provided to enclose the various components and environment which is necessary to sputtering. Many other configurations such as closed tubes or the like might be used provided of course that a gaseous plasma can be developed and provided the container, whatever the shape may be, is vacuum tight. The envelope 11 is connected to an electrically insulating base member 12. The base 12 is provided with a large diameter conduit 13 which connects the interior of chamber 14, formed by envelope 11 and base 12, to a mercury diffusion pump (not shown). The chamber 14 may also be provided with a gas inlet 16. The envelope may be made of any material such as glass. It is secured to the base member 12 by well known means which may include bolting the envelope 11 to the base member 12, clamping the members together or similar means. The important requirement is that the two members be securely attached to prevent a loss of vacuum inside the envelope 11 and to maintain a controlled supply of gas which is to provide the glow discharge'plasma within the tank 14.

Mercury gas is provided for the tank 14 from the mercury pool 17. This pool provides a concentration adequate to maintain a glow discharge plasma which will efliciently accomplish sputtering of the target material and other material within the envelope 14. Instead of mercury, a number of other gases such as any of the noble gases including helium, neon, argon, krypton, and xenon may be utilized as the gas for producing the plasma. Other gases might be used such as hydrogen or any gas which is capable of being ionized and which is capable of producing positive ions for bombardment of a target material. For purposes of illustration, however, the use of mercury gas will be utilized. If these other gases are used, they may be introduced into the tank 14 through a gas inlet 16. In this case, it may be desirable to remove some of the mercury gas from the tank 14 by freezing it out or by using some other ignitor and plasma discharge system.

Next, the gas must be ionized to form the glow discharge plasma containing positive ions. As previously noted, any number of methods might be utilized to light or start the gas to form the plasma. As an example, one might use an Rf source to produce the glow discharge. For purposes of illustration of the present invention, a DC potential is applied between a cathode which is illustrated by mercury pool 17 and anode 18. A striker electrode 28 is used in the mercury pool 17 to start the glow discharge to form the plasma. A typical voltage which might be utilized to maintain the glow discharge may be in the order of about 20 volts DC between anode 18 and the mercury pool 17 which acts as the cathode. Other voltage ranges may be desirable and may be utilized depending on the ion density desired, type of gas used, the pressure of the system etc. However, it has been found that this particular voltage range works satisfactorily when mercury gas is used. An auxiliary anode 26 and lead 27 are provided to insure efficient starting of the glow discharge. The anode 26 is situated near the striker 28. An example of this type of DC system may be readily referred to in copending application Ser. No. 134,- 458, filed Aug. 28, 1961, now abandoned by Gerald S. Anderson et al. titled, Method and Apparatus for Ionic Sputtering. This copending application illustrates in greater detail the apparatus utilized to maintain a glow discharge with a DC potential.

The anode space and the cathode space of the tube or tank 14 are separated by a fine mesh grid electrode 20 positioned in plate 25. Although it is not essential, the grid 20 helps to stabilize the main discharge and permits considerable increase in plasma density within the anode space without the use of undesirable high discharge currents. It also provides a simple, yet effective, control of the velocity of accelerated beam electrons by variations of the grid potential.

Next, the oxide body 21 which acts as a target material must be situated within the envelope 11 so that it is in the plasma developed by the glow discharge. There is no critical position or location for the target 21 within the envelope 11 except that it must be situated where the positive ions developed in the plasma may bombard the surface 22 of the oxide with adequate concentration to remove oxygen atoms from the surface 22 of the oxide body 21.

A glass protector 23 is utilized to hold the oxide body or base 21. This glass protector 23 acts as a shield for the lead 19 and the metal electrode 24 upon which the oxide body 21 is mounted. The lead 19 to the metal electrode extends through the wall of the envelope 11 and extends outside where a connection may be made to a source of alternating potential such as a radio frequency (RF) source. The electrode 24 and lead 19 are mounted within the glass protector 23 so that the lead 19 and electrode 24 will not be bombarded and thus destroyed by ionized particles from the plasma developed within the chamber 14. The oxide body 21 is secured against the electrode 24 so that the oxide body will be energized to establish the potential field necessary to attract positive ions from the plasma so that they strike surface 22.

The oxide body 21 may be any metal oxide. It may also be formed of such oxides as silicon oxide or the like. As an example, it may be barium titanate, titanium dioxide, tantalum oxide, copper oxide, iron oxide or any other metal oxide as well as semiconductor oxides.

The alternating potential circuit is completed through a anode 18. This anode 18 is positioned within the chamber 14 to complete the circuit within the chamber 14 as well as providing a circuit for the DC potential utilized for the glow discharge. An alternating source is not essential to the practice of this process since the only condition which is essential is to maintain the surface 22 of oxide body 21 at a negative potential in respect to the plasma. This might be accomplished, for example, by simply applying a DC voltage between leads 19 and 30. It has been found, however, that the application of an AC potential, particularly in'the RF range, works very effectively when the body 21 is a nonconductor. The AC potential prevents a build-up of a positive charge at surface 22 which would ordinarily reduce the potential difference between surface 22 and the plasma. This application of the preferred RF potential to the target is more fully explained in the above identified copending application.

After the oxide body 21 is positioned within the area where the plasma is developed, the sputtering or bombardment of the surface 22 by ions may be accomplished. First the gas, such as mercury gas, is introduced into the envelope 11 either from mercury pool 17 or through the gas inlet 16. Next the mercury gas is lighted or started so that a glow discharge plasma is developed in the area within the envelope 11. This is accomplished by applying a DC voltage between anode 26 and the starter electrode 28 and applying a DC potential to anode 18. Starter electrode or cathode 28, anode 18 and anode 26, provide the electrical circuit for starting and maintaining the glow discharge in the chamber 14. The oxide body 21 is next energized by application of RF energy between electrodes 18 and 24. This radio frequency (RF) energy induces sputtering of the surface 22 of the oxide body 21.

Refer now to FIGURE 2 of the drawings, which shows a cross section of the oxide body 21 in enlarged cross sec tion. The potential which is applied to the oxide body 21 is used to make surface 22 negative with respect to the plasma potential. The result is a development of a Langmuir sheath which develops near the negative surface 22 of the oxide body 21 or target. The Langmuir sheath is represented between lines 29 and 31. Reference is made to an article by G. S. Anderson, William N. Mayer and G. K. Wehner titled, Sputtering of Dielectrics by High Frequency Fields published in the Journal of Applied Physics, vol. 33, No. 10, pages 2991-2992, published Octics of the Langmuir sheath. Within this area, designated tober 1962, for an explanation of some of the characterisas the Langmuir sheath, there is a deficiency of electrons since surface 22 is negative with respect to the plasma. Within this sheath, the electrons are repelled from the surface 22 of the body 21 into the plasma and only positive ions are present. These positive ions are developed in the plasma and are attracted to the surface 22 by the voltage (RF potential) which is applied to the oxide body 21. These positive ions strike the surface 22 of the oxide body 21. When this collision occurs, oxygen atoms are dislodged from the surface 22 of the oxide body 21 and leave a deficiency of oxygen atoms in a thin layer 32 at the surface 22. The dislodged oxygen atoms are actually negative ions which leave the surface 22 and due to the presence of the Langmuir sheath are repelled away from the surface 22 beyond the boundary 31. The result of this discharge or dislodgment of oxygen atoms which are in combination with a metal, for example, results in the production of a layer 32 which is primarily metal which formerly was combined with the oxygen to form the oxide of the metal. The metal remaining is a part of the atomic structure of the oxide and consequently is securely bonded to the oxide body 21.

A complete understanding of the theory of this bombarding process which dislodges the oxygen atom is not understood, however it is believed that the atomic bond between the oxygen and the metal of the oxide body 21 is broken with the result that negatively charged oxygen atoms are released from the atomic bond. On release of the negatively charged oxygen atoms, they are accelerated away from the surface 22 since the surface 22 is negatively charged. The released oxygen atoms from the surface 22 results in an oxide layer 32 which is metal enriched.

Since the oxide body :21 is essentially a non-conductor, alternating potential or RF potential is required to effectuate an efiicient bombardmentof the body 21 by the positive ions 33. As previously noted, the alternating potential continuously discharges any build-up of positive ions at the surface 22 which would result in a reduced efficiency of the bombardment process. An explanation of the use of an alternating potential is more fully set forth in the above-identified copending application and reference to that copending application is made for an explanation of the need for alternating energy for the oxide body 21.

The surface 22 of the oxide body 21 may be bombarded by the ions 33 for a sufficient length of time to produce a thickness in the layer 32 which may be desired for further utilization. If the surface 22 is bombarded a suflicient length of time, the layer 32 is enriched to the point that the surface 22 is essentially all metal and contains very little oxygen. Although the bombarding positive ions 33 will dislodge some metal atoms from the surface, it is noted that the oxygen atoms tend to be dislodged in preference to the metal atom with the resulting concentration or enrichment of the surface 22 with metal atoms. Thu-s it can be seen that the surface 22 of the metal oxide body or base 21 is provided with a layer 32 of metal which forms a part of the atomic structure of the oxide body 21.

The metal composing the layer 32 retains essentially the characteristics of an ordinary layer or sheet of metal or material of the type forming the oxide and can be treated, processed, and utilized accordingly.

The oxide body may be coated with a metal without removing the oxide body from the plasma. Refer now to FIGURE 3 of the drawing for an explanation of the process for effectuating this coating. The electrodes needed for increasing the metal concentration at the surface 22 of the oxide body 21 are positioned substantially the same as in FIGURE 1 of the drawings and the conditions necessary for the metal plating are the same as that established for the process involved in FIGURE 1. One additional electrode is needed to accomplish the metal plating of the oxide body 21. This electrode is illustrated by electrode 34 in FIGURE 3. Electrode 34 is connected to terminal 36 which extends through the envelope 11. A body of metal which is to be plated on the surface 22 or layer 32 of the oxide 21 is placed in contact with electrode 34 so that the metal body 37 may be energized.

Up to the point where the metal body 37 is bombarded by positive ions from the plasma, the process is the same as that previously described for simply removing oxygen atoms from the surface 22 of the oxide body 21 to develop layer 32. The glow discharge plasma is lighted by application of voltage to the striker 28. As previously noted, this may be a DC potential or some other system might be utilized such as RF energy to start and maintain the glow discharge necessary to the process. Next, a potential, preferably an alternating potential, is applied between the electrode 24 and electrode 18. This alternating potential might be, for example 1600 volts AC which is carried on a negative 800 volts DC. The negative 800 volts DC is a result of self biasing by the system. See the above reference article for more detail concerning the biasing. The 1600 volts AC is a peak to peak voltage and is merely illustrative of a voltage which might be utilized in the system. With these conditions, the surface 22 of the oxide body 21 is bombarded by positive ions from the plasma and oxygen atoms are removed from the surface 22 of the oxide body 21, a metal oxide, for example, leaving the surface 22 enriched in the metal of the metal oxide body 21.

After the surface 22 has been bombarded sufiiciently to increase the concentration of the metal at the surface 22, the next step involves deposition of the metal from metal body 37 on the surface 22 or layer 32. While the oxide body 21 remains energized by the alternating potential, a DC voltage is applied to terminal 36. This voltage may be, for example, a negative 400 to 800 volts DC and is negative with respect to the alternating current ground (plasma reference). Now, a number of the positive ions from the glow discharge plasma are attracted to the metal body 37 and strike the metal body with sufficient energy to dislodge neutral atoms of the metal composing the body 37. These atoms diffuse throughout the container or envelope 11 and many of them come in contact with the surface 22 where they are deposited on the surface. Since the surface 22 is primarily a metal enriched surface, the diffused metal atoms which contact this surface 22, form a bond with the metal of the oxide body 21. Since the surface 22 is metal, a strong atomic bond is formed between the coating or depositing metal atoms or ions and the metal of the oxide body 21. Thus it can be seen that in effect the deposited metal essentially forms a part of the atomic structure of the oxide body 21 due to the fact that there is a metal deposition on a second metal which is a part of the atomic structure of the oxide body 21. It has been found that such a film of deposited metal is extremely uniform, provides a strong bond to the metal oxide body 21 and provides other desirable characteristics of metal to metal deposition. The depositing metal 37 might be any of the known metals which might be desired as a coating material on a particular oxide 21. Providing metal to metal bonds is well known in the industry and the range of metal depositions or films which may be deposited on a particular metal is only limited by the number of metals available and the particular characteristics of the combining metals which might be desired.

It is to be understood that the above described arrangements are simply illustrative of the application and the principles of the invention, and many other modifications may be made by those skilled in the art without departing from the spirit or scope of the invention.

Now therefore I claim:

1. The process for depositing an adherent metal coating on a surface of an oxide body, said oxide body being taken from the group consisting of barium titanate, copper oxide, iron oxide, silicon oxide, tantalum oxide and titanium dioxide, which comprises:

placing both said oxide body and a piece of said metal in an enclosure;

feeding a gas taken from the group consisting of argon,

helium, krypton, mercury, neon and xenon into said enclosure;

ionizing said gas to produce a plasma in said enclosure; applying a radio frequency signal to said oxide body to bombard said surface thereof with gas ions and selectively remove oxygen from said surface; and

applying a potential to said piece of metal to sputter atoms of said metal for deposition on said surface of said body.

2. The process of depositing a met-a1 coating on a surface of an oxide body, wherein said oxide body is taken from the group consisting of barium titanate, copper oxide, iron oxide, silicon oxide, tantalum oxide, and titanium dioxide, which process comprises:

mounting said oxide body and a piece of said metal within an enclosure;

feeding a gas taken from the group consisting of argon,

helium, krypton, mercury, neon and xenon into said enclosure;

ionizing said gas to provide a plasma in said enclosure;

applying a radio frequency signal to said oxide body for bombarding said surface of said oxide body with gas atoms to selectively remove oxygen from said surface; and

coating said bombarded surface with particles of said piece of metal.

References Cited UNITED OTHER REFERENCES Anderson et a1.: J. of Applied Physics, vol. 33, N0. 10, pp. 2991-2.

Holland: Vacuum Deposition of Thin Films, 1956, 5 Chapman & Hall Ltd., London, pp. 74-80.

Wehner: Advances in Electronics and Electron Physics,

STATES PATENTS Brunke et a1. l17106 X Chilowsky 0 vol. VII, 1955, Academlc Press Inc. Pub., New York, Holland 117-933 X PP- g,, 10 JOHN H. MACK, Primary Examiner.

Lubin 204192 R. K. MIHALEK, Assistant Examiner. 

2. THE PROCESS OF DEPOSITING A METQAL COATING ON A SURFACE OF AN OXIDE BODY, WHEREING SAID OXIDE BODY IS TAKEN FROM THE GROUP CONSISTING OF BARIUM TITANATE, COPPER OXIDE, IRON OXIDE, SILICON OXIDE, TANTALUM OXIDE, AND TITANIUM DIOXIDE, WHICH PROCESS COMPRISES: MOUNTING SAID OXIDE BODY AND A PIECE OF SAID METAL WITHIN AN ENCLOSURE; FEEDING A GAS TAKEN FROM THE GROUP CONSISTING OF ARGON, HELIUM, KRYPTON, MERCURY, NEON AND XENON INTO SAID ENCLOSURE; IONIZING SAID GAS TO PROVIDE A PLASMA IN SAID ENCLOSURE; APPLYING A RADIO FREQUENTLY SIGNAL TO SAID OXIDE BODY FOR BOMBARDING SAID SURFACE OF SAID OXIDE BODY WITH GAS ATOMS TO SELECTIVELY REMOVE OXYGEN FROM SAID SURFACE; AND COATING SAID BOMBARDED SURFACE WITH PARTICLES OF SAID PIECE OF METAL. 